1. Field of the Invention
The present invention relates to a display apparatus which includes an active element for driving a self-emissive element such as an organic EL (ElectroLuminescent) element, LED (light emitting diode) or the like, and method of driving the same, and more particularly to a display apparatus which includes a TFT (thin film transistor) using an organic semiconductor as the active element.
2. Description of the Related Art
The TFT is widely used as an active element for driving an active matrix type display such as an organic EL display or a liquid crystal display. FIG. 1 depicts a diagram showing an example of an equivalent circuit for driving, for example, an OLED (Organic Light Emitting diode) 100 which is an organic EL element. Referring to FIG. 1, this equivalent circuit includes a capacitor CS, and two p-channel TFTs 101, 102 which are active elements. A scanning line WS is connected to a gate of the selection TFT 101, a data line WD is connected to a source of the selection TFT 101, and a power supply line WK for supplying a constant supply voltage VDD is connected to a source of the driving TFT 102. The selection TFT 101 has a drain connected to a gate of the driving TFT 102, and a capacitor CS is formed between the gate and the source of the driving TFT 102. The OLED has an anode connected to a drain of the driving TFT 102, and a cathode connected to a common potential, respectively.
As a selection pulse is applied to the scanning line WS, the selection TFT as a switch turns on and therefore has a conducting channel between the source and the drain. At this time, a data voltage is supplied from the data line WD through the source and drain of the selection TFT 101, and charges are accumulated to create the data voltage on the capacitor CS. The data voltage created on the capacitor CS is applied between the gate and source of the driving TFT 102, thus causing a drain current ID to flow in accordance with a gate-source voltage (hereinafter referred to as the “gate voltage”) Vgs of the driving TFT 102. The drain current ID is supplied to the OLED 100. However, a threshold voltage of the driving TFT 102 shifts as the driving time passes. An example of the relationship between the gate voltage Vgs of the TFT and the drain current Id is shown in FIG. 2. As shown in FIG. 2, there can be found a phenomenon that a curve 120A in an initial state shifts to a curve 120B as the driving time passes, and that the gate threshold voltage shifts from Vth1 to Vth2. Such a threshold voltage shift causes problems of giving rise to a reduction in luminance of light emitted by the OLED, and making the TFT inoperative.
Single crystal silicon, amorphous silicon, polycrystalline silicon, or low-temperature polycrystalline silicon is widely used for an active layer which forms part of the TFT. In recent years, attention has been paid to a TFT which uses an organic material that is based on carbon and hydrogen as an active layer (hereinafter referred to as the “organic TFT”), instead of those silicon materials. FIG. 3 depicts a schematic diagram showing a cross section of a typical organic TFT. This organic TFT includes a plastic substrate 111, a gate electrode 112, an insulating film 113, a drain electrode 114, a source electrode 115, and an organic semiconductor layer 116. The gate electrode 112 is formed on the plastic substrate 111, and the insulating film 113 is formed to cover the gate electrode 112. On this insulating film 113, the drain electrode 114 and the source electrode 114 are deposited so as to oppose each other, and the organic semiconductor layer (i.e., active layer) 116 is formed between the drain electrode 114 and the source electrode 115. Materials used for the organic semiconductor layer 116 include low-molecular-weight based or polymer based organic materials having a relatively high carrier mobility, such as pentacene, naphthacene, or polythiophen-based materials. This type of organic TFT can be formed on a flexible film such as a plastic film in a relatively low-temperature process, and therefore enables a mechanically flexible, light weight, and thin display to be readily manufactured. Also, the organic TFT can be formed by a printing process, or a roll-to-roll process at a relatively low cost. The aforementioned threshold voltage shifting phenomenon appears conspicuously in the amorphous silicon TFT and organic TFT. The threshold voltage shift of the organic TFT is disclosed, for example, in the following article: S. J. Ziler, C. Detcheverry, E. Cantatore, and D. M. de Leeuw, “Bias stress in organic thin-film transistors and logic gates,” Applied Physics Letters Vol. 79(8), pp. 1124-1126, Aug. 20, 2001.
Driving circuits and driving methods which can compensate for a threshold voltage shift of the TFT are disclosed, for example, in Japanese Patent Application Kokai No. 2002-514320 (corresponding to U.S. Pat. No. 6,229,506), and Japanese Patent Application Kokai No. 2002-351401 (corresponding to U.S. Patent Application Publication No. 2003112208). Either of the driving circuits and driving methods described in these documents can control the OLED to emit light at a constant luminance, while accepting the threshold voltage shift of the driving TFT. However, since the driving circuits of these documents cannot either eliminate the threshold voltage shift, they cannot prevent an increase in power consumption due to the threshold voltage shift. Also, if the threshold voltage of the driving TFT shifts beyond an allowable range, it is difficult to compensate for the shift, resulting in variations in the luminance of light emitted by the OLEDs, and in inoperative TFTs. Further, since the threshold voltage shift occurs in the selection TFT as well, other than the driving TFT, the selection TFT will be made inoperative if the threshold voltage of the selection TFT shifts beyond the allowable range. Particularly, the threshold voltage shift of the organic TFT is large as compared with those of the low-temperature polysilicon TFT and single crystal silicon TFT, so that an active matrix type display which uses the organic TFT has a problem of higher susceptibility to variations in the luminance of light emitted by the OLEDs, and inoperative TFTs.